A non-destructive optical technique for testing lacquered metal surfaces

A simple non-destructive optical technique is described for testing lacquering on metal. A d.c. voltage is applied across a nematic liquid crystal layer sandwiched between the lacquered surface under investigation and a transparent conductive layer on a microscope slide. Current leakage across the nematic as the voltage is raised disturbs the molecular structure in the liquid crystal and causes dynamic optical displays viewed through the glass slide. Leakage occurs readily at pin-holes, less readily at physical or chemical heterogeneities in the lacquer. Examples are given of the application to defects in lacquer on tin-plate.